Let me add this question (which is what I thought the OP was asking): If they already carry the HD, why not just downconvert that instead of taking up bandwidth transmitting both the HD and SD versions of the same channel? Isn't that feasible?
His was different as I read it, but to respond to yours:
Simply carrying the HD feed and "down-converting" it in route to the receiver would probably not free anything up, at all. In fact, it might possibly increase the bandwidth used. So this option would be out, I think.
In order to 'Free the Bandwidth,' (wouldn't that be a great T-Shirt?-Yes, I will accept a nickle for every shirt sold and probably buy one myself) the box would have to "down-convert" it, and that requires "retooling" boxes to handle it. First, they have to be tooled up to actually see the channels (software), and then they have to have a "processor" of some sort to use a reverse version of line doubling or line averaging to make a clear picture on an SD set rather then have a jigsaw edge to the picture.
The amount of free bandwidth is limited locally to DMA (not even regionally, in case anyone was thinking RSNs), and may not matter much in the grand scheme of things. All of this comes to a point where cost, feature, and benefit may not be practical, whether feasible or not.
My question is: why carry both?
It could simply be "customer satisfaction". Much like those of us with HDTVs get annoyed with stretching a 4X3 picture to fit a 16X9 screen, there are people with the opposite problem. They are annoyed by a smaller "widescreen" 16X9 picture rather then filling their 45 inch big screen with a 4X3 format. A lot of 4X3 sets don't have a picture format or zoom feature on them

eek: its hard to imagine). SD boxes don't have that feature either.
It also stands to reason that if HD is not included in a contract and therefore is not allowed to be carried, that the exact reverse could be true: SD is in the contract and
required to be carried.
Good question, but no real 100% guarantee of an answer.
All in all, remember that digital is the standard, not HD. While there may be a lot of people nationwide that have a single HDTV in the home (in fact, I think HDTV penetration into the market is well over 60% percent (at least an article I found from May of last year says so), it is not every home, and some homes have more TVs and simply the one capable of HD. That figure is also the national "tally" and not isolated just to DISH subscribers (which could show a completely different ratio of HD and SD customers).